MAIN FEATURE
STORM OVERFLOW REDUCTION
Challenges and opportunities for the water industry
There has been a lot of discussion about reducing the discharge of untreated sewage from overflows; but what are
the changes in the rules and what are the implications for the water companies and their supply chains.
Storm overflows are safety valves built into the combined sewerage system to discharge excess flows of dilute sewage to rivers, lakes, or the sea when rainfall exceeds capacity. This protects properties and other areas from flooding. They are also provided at wastewater treatment works to protect the works from damage to their operation from excessive flows. The overflows at treatment works generally have storage tanks to capture the discharge and store it for treatment later.
History of the rules
The rules for overflow design have always included limiting their discharge so that they do not cause harm to the water environment rather than to a specific frequency. The rules for protecting the health of bathers and other recreational water users were introduced later and are not considered in this article.
The original work about a hundred years ago set limits on pollutant concentrations in treated effluent based on what would not cause harm to the receiving water. Guidance for overflows at this time was that they should not spill
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until the flow in the sewer was diluted 6 times with rainfall. This would mean that it was almost as dilute as treated effluent.
In 1970 a detailed study led to an improved approach called Formula A, which increased the required dilution in the sewer to between 6 and 8 times. The guidance said that this was appropriate if the dilution ratio (low river flow divided by foul sewer flow) was 8 or more. For lower ratios, greater dilution would be required in the sewer; but the guidance didn’t say how much. A typical Formula A overflow will spill approximately 40 times a year.
In 1977 the so-called Scottish Method defined the additional requirements for low river dilution.
In 1994 the Urban Pollution Management (UPM) manual changed from rules of thumb to a method for a detailed analysis of the impact of each overflow on the ecological health of the receiving water. This took into account the dilution in the sewer and the dilution in the river. The studies were complex and expensive but over the next 10 or 15 years this approach was used to deliver massive
| February 2025 |
draintraderltd.com improvements in river water quality.
History of the law Discharges from each overflow were legally allowed as long as the overflow was designed, constructed and operated in line with its discharge permit. These permits would be based on one of the assessment methods described above. Unfortunately these rules were not always applied correctly and systems were not kept up to date with population growth, so a lot of overflows did cause environmental harm.
In 1991 the European Urban Wastewater Treatment Directive (and UK regulations UWWTRs) set out new requirements for controls on overflow discharge. These included that they should minimise spills and limit environmental impact.
“Such measures could be based on dilution rates or capacity in relation to dry weather flow, or could specify a certain acceptable number of overflows per year.”
The UK dilution approach was therefore accepted in the regulations. However some years later the UK was challenged
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